In a brake apparatus of a vehicle, such as an automobile, when a driver depresses a brake pedal, braking operation is performed. In the case of a brake apparatus in which braking pressures for wheels are controlled on the basis of the magnitude of braking operation by the driver, generally, the rate of increase of an operational reaction force which the driver feels gradually increases with the magnitude of braking operation by the driver. Thus, the magnitude of braking operation by the driver (brake stroke) and the operational reaction force (brake reaction force) are preferably in a nonlinear relation.
Conventionally, there have been proposed brake stroke simulators of various configurations which achieve a nonlinear characteristic of brake stroke vs. brake reaction force. For example, Japanese Patent Application Laid-Open (kokai) No. 2005-112034 describes a brake stroke simulator configured to be able to adjust a nonlinear brake stroke vs. brake reaction characteristic.
A conventional brake stroke simulator in which a brake stroke and a brake reaction force are in a nonlinear relation generally has a plurality of reaction force generation springs whose spring constants are fixed and differ from one another. In generation of reaction force, the different reaction force generation springs are individually selected according to a brake stroke, thereby achieving a nonlinear characteristic. Accordingly, the relation of the brake reaction force to a brake stroke is of discontinuous nonlinearity; in other words, the relation fails to exhibit preferred continuous nonlinearity.
In order for the relation of the brake reaction force to the brake stroke to be of continuous nonlinearity, a single reaction force generation spring having a nonlinear spring characteristic can conceivably be used. However, it is very difficult to form a reaction force generation spring which imparts preferred continuous nonlinearity to the relation of the brake reaction force to the brake stroke over the entire range of brake stroke.
The above-mentioned problem in relation to a brake stroke vs. brake reaction force nonlinear characteristic is not limitingly involved only in the brake stroke simulator, but is also involved in other manipulation simulators, such as a flight simulator, in which operation means is operated by an operator and which allows the operator to operate the operation means and applies an operational reaction force to the operator via the operation means.